Current antilock braking systems employ, between a source of pressurized fluid and a pressure receiver such as a brake motor, a solenoid valve commanded by a computer on the basis of signals representing the speed of rotation of the wheels in order, in general, to relieve the pressure of the fluid in the brake motor when the computer detects imminent locking of the wheel associated with this brake motor, then to connect a second source of pressurized fluid in order to increase the hydraulic pressure in the brake motor again until imminent locking is detected again, the pressure-relief and -raising cycle then recommencing.
Likewise, known traction control systems employ a solenoid valve in order, in general, to increase the pressure of the fluid in the brake motor when the computer detects the need to brake the wheel associated with this brake motor, then to connect a second source of fluid under low pressure in order to relieve the hydraulic pressure in the brake motor until braking is necessary again, it being possible for the pressure-raising and -relieving cycle to recommence.
The solenoid valves used to command the braking pressure operate most of the time in all-or-nothing mode, one solenoid valve being used to relieve the pressure in the brake motor, and another to make the pressure in this brake motor rise again, this being for each wheel of the vehicle.
These successive phases of relieving the pressure and raising it again each last for a very short period of time and follow on rapidly from one another, which results in the solenoid valves changing state rapidly many times. This gives rise to significant noise due to the beating of the movable part of these solenoid valves, accompanied by abrupt changes in pressure in the brake motors, giving rise to transient ill-controlled conditions.
In order to avoid these drawbacks, it has since been proposed, for example in the documents FR-A-2,679,299 or FR-A-2,683,338, to use three-way solenoid valves of the proportional type. They allow, on the one hand, use of just one solenoid valve per wheel of the vehicle to be commanded and, on the other hand, they operate much more quietly.
Such proportional solenoid valves nevertheless still have the drawback of exhibiting a significant overall size. Indeed, these solenoid valves are made up most of the time of a purely electrical part including the electrical coil, the pole pieces and a movable magnetic core, and a purely hydraulic part including a distributor slide valve sliding in a bore or in a sleeve formed in a body including various hydraulic ducts, the connection between these two parts being effected by a push rod integral with the movable magnetic core on which the distributor slide valve comes to bear.
Such an in-line arrangement of the electrical and hydraulic parts gives known solenoid valves longitudinal dimensions which may make it difficult, or even impossible, to install them in the event of the space given over to them being too limited.